Normally a case can only be registered to a customer being an account or Contact. This will normally do the trick since you know the customer, when he registering a case. But in some organizations there are thousands of leads and they don’t want to combine the leads with contacts or accounts.
The solution it then to create a new relationship between case and Lead. and depending on what you want to do with the history linked to a lead when the lead converts to a account/contact and opportunity, a plugin is required to link the case to the appropriate account or contact.